\[- 75b^{6} + 30b^{4} - 3b^{2} =\]
\[= - 3b^{2}\left( 25b^{4} - 10b^{2} + 1 \right) =\]
\[= - 3b^{2}\left( 5b^{2} - 1 \right)^{2} =\]
\[= 3b^{2}(5b^{2} - 1)(1 - 5b^{2})\]